Position sensors are required for modern automobile engine control systems to detect the angular position of the crankshaft. A vehicle's engine controller uses this information to calculate the optimal times to fire fuel injectors and, for spark ignited engines, ignition discharge. The information is also useful for diagnosing malfunctions such as misfire, which typically is done by detecting a rapid change in angular velocity.
Prior art sensors for measuring rotary position typically incorporate encoders, resolvers, or potentiometers. In the hostile environment of automotive engines, incremental encoders of some form are typically used. Other types of position sensors are typically analog in nature and have poor noise immunity and/or drift with temperature changes, and sometimes have wear-prone brushes. Encoders have the advantage of being simple and reliable even when subjected to temperature extremes.
They require only a single timer/counter channel of a vehicle's controller to function and therefore are also inexpensive.
An incremental encoder is typically constructed as a series of spaced-apart features such as alternating teeth and notches around the circumference of a wheel connected to a rotating device such as a crankshaft. The wheel may be formed as a part of the apparatus, for example, an engine flywheel may be formed as an encoder wheel. As the wheel spins, a pickup sensor mounted in close proximity to the wheel detects each feature as it passes by the pickup, for example, by chopping of an optical beam or variation in magnetic field, and generates a square wave response. An index pulse function is typically present to identify an absolute or reference position of the wheel, such as top dead center of the No. 1 engine piston. This sometimes consists of two features more closely spaced than the other features. The reference position also may be established externally by a feature on an engine camshaft which is synchronized to the crankshaft at one-half its angular velocity.
A known problem exists in using prior art systems when starting an engine. The index feature must be found in order to synchronize the engine control system, which can require up to one full revolution of the engine to locate the index feature on the crankshaft or two full engine revolutions to locate the index feature on the camshaft. This represents time delay in starting the engine.
Although prior art absolute encoder technology is well known, such an encoder system requires multiple inputs in a controller for each bit of resolution desired. For example, an encoder with three bits requires a more complex wheel, three wires, connections, and inputs, adding to the cost while decreasing the reliability. An encoder with three bits of resolution provides an angular resolution of 360/23 degrees, equals 45 degrees.
Another prior art technology uses a series of unevenly spaced features and pulses. By searching for a pattern of spacing, the position can be established in this fashion. Such a system typically requires a plurality of pulses before the position can be identified. Although this may be an improvement over waiting for an index pulse, this system typically requires up to one-half an engine revolution (180 degrees) before the position can be established.
What is needed in the art is a method and apparatus for determining rotary position of a rotatable element in a small fraction of a single revolution, preferably within 45 degrees or less.
It is a principal object of the present invention to determine the rotary position of a rotatable element within a small fraction of a single revolution of the element.
It is a further object of the invention to minimize the time required for an internal combustion engine control system to determine the angular position of the engine at start-up.